The acetaldehyde content of polyester beverage bottles is of particular importance. Acetaldehyde is created in small amounts due to a thermal decomposition reaction. When using polyester for foodstuff packaging, in particular in connection with bottled beverages, traces of acetaldehyde are bothersome, because acetaldehyde, which is a very odor- and taste-intensive substance, noticeably changes the taste. An acetaldehyde concentration of 3 .mu./l, measured in the gas content of a newly produced closed polyester bottle after 24 hours, was set as the upper acceptable limit by the Coca Cola Company (Coca Cola standard.)
To meet these requirements, i.e. reduce the level of acetaldehyde to an acceptable level, it has been necessary in accordance with the prior art to subject the raw polyester initially produced in the melt phase to a special solid phase treatment.
This state of the art is described in the conference proceedings to "Entgasen beim Herstellen und Aufbereiten von Kunststoffen" [Devolatilizing in the Course of Producing and Processing Plastic Materials] in an article by A. Furst "Entgasen bei der Polykondensation" [Devolatilizing in the Course of Poly-Condensation], pp. 187 and 192, VDI-Vedag GmbH, publ., Dusseldorf, 1992. The conventional route via the solid phase includes the steps of granulation of a medium-viscous melt, crystallization of the amorphous polyester granules and solid phase poly-condensation in order to obtain granules with a higher viscosity suitable for bottle production and a low acetaldehyde content (in the range of approximately 1 ppm).
Besides the fact that with the conventional process the heat content of the polyester is lost twice so that energy requirements are high, the solid phase treatment is demanding and expensive because of the stickiness problem. The crystallization in particular of copolyesters preferred for bottle production, which tend to stick together even more strongly than normal (homo-)PET, requires special methods and devices. An example of this is provided in EP 0 379 684 B1, wherein two fluidizing devices are used. But even in a conventional solid phase polycondensation reactor, which is operated at even higher temperature than the crystallizer, there is no immunity against stickiness which can lead to a stoppage of the plant. Proposals for overcoming this problem have been made, for example, in EP 0 269 583 B1 and in WO 94/17122. It is furthermore proposed in EP 0 541 674 B1 to give the granules a cross-sectional "dog bone" shape, based on the conside-ration that granule bodies of such shape can only touch at a few points and that thereby the danger of agglomeration is reduced during solid state treatment.
In accordance with the above process it is therefore possible to produce bottle granules (bottle-grade chips). It is then necessary to transport these granules to the producer of preforms or bottles.
The production of the bottle from the bottle-grade granules takes place in two process steps, such as described, for example, in "Industrielle Verfahren zur Herstellung biorientierter Hohlkorper" [Industrial Processes for Producing Bi-Oriented Hollow Bodies] by M. S. Merlini in Kunststoffe/Plastics 3/83, pp. 17 to 20. The solid-phase post-condensed polyester granules are added in the first step, since they have again absorbed moisture during the delivery, are next melted in the extruder of an injection molding machine and pressed into the cavities of the injection molding tool, whereby preforms of the subsequent bottles are obtained. In the second process step the preforms are stretch-blow molded at approximately 100.degree. C. and bi-axially (radially and axially) oriented during blow-up, thus giving the finished bottle the required use properties. These two process steps can usually be performed completely separately, between which steps the preforms are completely cooled.
During melting of the granules in the injection molding extruder, the acetaldehyde content in the polyester again increases because of shearing action and high temperatures, because some thermal decomposition and thus new formation of acetaldehyde will take place. With optimally adjusted extruders and injection molding units, the extent of this new formation of acetaldehyde increases by approximately 5 ppm which, together with the initial value of the bottle-grade granules (approximately 1 ppm) results in an acetaldehyde content in the preform of approximately 6 ppm. An approximately proportional correlation exists between the acetaldehyde content in the preform and the acetaldehyde concentration in the finished bottle. With 1.5 liter disposable bottles (of a weight of approximately 48 g), experience has shown the following relationship to apply: acetaldehyde concentration in the finished bottle (expressed in .mu.g/l, related to the gas volume respectively the contents of the bottle)=0.3.times.the acetaldehyde content in the preform (expressed in ppm, related to the mass of the polyester). This means that from a preform with 6 ppm acetaldehyde content a corresponding value in the bottle of approximately 1.8 .mu.g/l can be expected. Looked at in the opposite way, the acetaldehyde content in the preform must not be higher than 10 ppm in order not to exceed the Coca Cola limit of 3 .mu.g/l.
Efforts have lately been made with respect to attempting to simplify the previously mentioned solid phase post-condensation of the granules used to injection mold the preform, again in order to reduce the acetaldehyde to an acceptable level.
EP 0 422 282 A1 describes a process wherein the dianhydride of an aromatic tetracarboxylic acid, preferably pyromellitic acid dianhydride, is metered and worked into a melt of PET or CoPET. The granules produced from this process show a very rapid increase of the intrinsic viscosity (I.V.) at relatively low temperatures during a solid phase poly-condensation. Because of this result it is possible to do without a solid phase reactor as previously mentioned. However, the granules still must be crystallized and dried, and the dianhydride has the further undesirable side effect that the second carboxyl groups, released after the addition reaction to the anhydride groupings, can lead at least partially to branching in a subsequent reaction, which worsens the flow properties of the melt during injection molding.
DE 43 09 227 A1 describes a process wherein, as above, the entire solid phase treatment can be performed in air because of the use of lower temperatures. In this case it was possible to reduce the process temperatures to within 185.degree. to 190.degree. C., because a finer than usual granulation is performed and the shorter diffusion paths or the greater specific surface result in a viscosity increase at lower temperatures and in an extensive removal of the acetaldehyde. This process is cumbersome and clearly of a longer duration than the conventional solid phase treatment and in addition, under the applied conditions of 14 hours in the air at temperatures up to 190.degree. C. (or 225.degree. C.), the granules are already being oxidatively attacked.